This invention was made with government support under Contract No. DEFC21-95MC31176 awarded by DoE. The government may have certain rights to the invention.
This invention generally relates to environmental coating systems for protecting metal substrates. According to some specific embodiments, the invention is directed to improved thermal barrier coating systems for metal components used in turbine engines. The invention also relates to processes for applying and treating such coating systems.
Many types of metals are used in industrial applications. When the application involves demanding operating conditions, specialty metals are often required. As an example, components within gas turbine engines operate in a high-temperature environment. Many of these components are formed from nickel-base and cobalt-base superalloys. Since the components must withstand in-service temperatures in the range of about 1100xc2x0 C.-1150xc2x0 C., the superalloys are often coated with thermal barrier coating (TBC) systems. These coating systems usually include a bond coat applied directly to the superalloy substrate, and a ceramic-based overcoat applied over the bond coat. For a jet engine, the coatings are applied to various superalloy surfaces, such as turbine blades and vanes, combustor liners, and combustor nozzles.
The effectiveness of a TBC system is often measured by the number of thermal cycles it can withstand before it delaminates from the substrate which it is protecting. In general, coating effectiveness decreases as the exposure temperature is increased. The failure of a TBC is often attributed to weaknesses or defects related in some way to the bond coat, e.g., the microstructure of the bond coat, or deficiencies at the bond coat-substrate interface or the bond coat-TBC interface.
The microstructure of the bond coat is often determined by its method of deposition. The deposition technique is in turn often determined by the requirements for the overlying protective coating. For example, many TBC""s are applied by a thermal spray technique, such as air plasma spray (APS). Coatings applied by such a method usually require a very rough bond coat surface for effective adhesion to the substrate. APS techniques are often used to provide such a surface for the bond coat.
While the APS process has several advantages, it also results in a porous coating microstructure. Such a microstructure allows significant internal oxidation of the bond coat. The oxidation of regions of the bond coat often reduces the concentration of aluminum in other bond coat regions. This phenomenon can in turn result in the diffusion of aluminum from an adjacent, aluminum-containing substrate, e.g., a superalloy. The depletion of aluminum from a superalloy substrate is especially severe when the component is used at the elevated temperatures described above. The loss of aluminum can be detrimental to the integrity of superalloy components.
In U.S. Pat. No. 6,165,628, problems associated with the microstructure of porous bond layers are addressed. In one embodiment of the reference, a bi-layer is used to bond a TBC to a metal substrate. The bi-layer includes a dense, primary bond layer over the substrate, and a xe2x80x9cspongyxe2x80x9d secondary bond layer over the dense layer. The primary bond layer is usually applied by a vacuum plasma spray (VPS) or high velocity oxy-fuel (HVOF) technique. The spongy, secondary bond layer is usually applied by APS. The primary bond layer helps to protect the substrate from excessive oxidation. The secondary bond layer promotes adhesion between the primary layer and the subsequently-applied TBC, while also acting as a strain-reliever between these two other layers. The resulting TBC system exhibits high integrity during exposure to high temperatures and frequent thermal cycles.
There is continuing interest in the development of other, improved coating systems which protect the substrate from excessive oxidation, while also providing environmental protection, e.g., thermal barrier properties. Systems in which the TBC adheres securely to an underlying bond coat are also very desirable. Furthermore, new coating systems which provide alternative methods for hermetically sealing the bond coat or the substrate would also be welcome in the art. Moreover, the TBC system should be very effective in protecting components used in high performance applications, e.g., superalloy parts exposed to high temperatures and frequent thermal cycles.
One embodiment of this invention is an article which comprises a metal-based substrate, and at least two layers overlying the substrate, wherein one of the layers is a coating which comprises a braze alloy, and another layer is a plasma-sprayed bond coat. The braze alloy often comprises a nickel-base or cobalt-base material, while the bond coat is often an MCrAlY-type material, where M is selected from the group consisting of Fe, Ni, Co, and mixtures of any of the foregoing. Moreover, the bond coat is often substantially porous, as described below.
The bond coat may lie on top of the braze alloy layer, or the braze alloy layer may lie on top of the bond coat. In the case of a porous bond coat (e.g., one applied by APS), partial or complete densification of the bond coat is sometimes carried out. The densification is achieved by heat treating the article, so that the braze alloy material migrates into the pores of the bond coat to a selected thickness. When the braze alloy layer is below the bond coat, the braze alloy material migrates upwardly into the bond coat. When the braze alloy layer is above the bond coat, the braze alloy material migrates downwardly into the bond coat. The article may further include a thermal barrier coating as the uppermost layer.
Another embodiment of this invention is directed to a method for providing environmental protection to a metal-based substrate, comprising the steps of applying a coating which comprises a braze alloy over the substrate, and plasma-spraying a bond coat over the substrate. As described previously, the two steps are interchangeable, and can be supplemented by the densification step. As used herein, xe2x80x9cenvironmental protectionxe2x80x9d refers to protection of a metal substrate from the adverse effects of oxidation, corrosion, and chemical attack. Thus, the processes claimed herein are especially suitable for protecting turbine engine components which may be exposed to extreme operating conditions.
Further details regarding the various features of this invention are found in the remainder of the specification.